The present invention relates to a variable venturi-type carburetor.
A variable venturi-type carburetor has been proposed in which a suction piston is slidably inserted into the casing of the carburetor and in which the interior of the casing of the carburetor is divided into an atmospheric pressure chamber and a vacuum chamber by the suction piston. The suction piston comprises a metering needle fixed onto the tip face thereof and a piston rod extending in a direction opposite to the extending direction of the metering needle. A hollow cylindrical support pipe, surrounding the piston rod, is fixed onto the casing of the carburetor, and the piston rod is supported by a linear ball bearing. This linear ball bearing comprises a plurality of aligned ball rows each extending in the axial direction of the piston rod and has such a construction that each of the balls is circulated in the support pipe. Therefore, the construction of the linear ball bearing is complicated, and when such a linear ball bearing is used, the cost of manufacturing the carburetor is disadvantageously increased.
An inexpensive bearing having the same function as that of the linear ball bearing is known in other industrial fields. As is illustrated in FIG. 10, this bearing comprises a hollow cylindrical sleeve a and a plurality of balls b and, thus, has a simple construction. Consequently, if this bearing is used, it is possible to reduce the cost of manufacturing the carburetor. However, if this bearing is used in the variable venturi-type carburetor illustrated in FIG. 10, a problem occurs in that the bearing moves towards the left in FIG. 10 due to a pressure difference between the vacuum in a vacuum chamber c and the vacuum in an interior chamber d. That is, if the engine speed is increased, the level of the vacuum in the interior chamber d becomes greater than that of the vacuum in the vacuum chamber c, and, as a result, the bearing moves towards the left in FIG. 10. FIG. 11 illustrates the relationship, obtained in the experiments, between the engine speed N and a pressure difference .DELTA.P which occurs between the vacuum in the vacuum chamber c and the vacuum in the interior chamber d. From FIG. 11, it will be understood that the pressure difference .DELTA.P is increased in accordance with an increase in the engine speed N. At the present stage, the reason why the pressure difference .DELTA.P occurs is unclear.
When the pressure difference .DELTA.P occurs, the bearing moves towards the left in FIG. 10, as mentioned above, and thus abuts against a snap ring e. However, if the bearing abuts against the snap ring e when the suction piston f moves towards the left in FIG. 10, the bearing is caused to move towards the right in FIG. 10 together with the suction piston f and thus the balls b do not rotate. As a result, since there is a delay in the movement of the suction piston f, a problem occurs in that the responsiveness of the suction piston to a change in the operating condition of the engine deteriorates. In addition, another problem occurs in that wearing of the balls is promoted.